Methods and Systems Employing Intracranial Electrodes for Neurostimulation and/or Electroencephalography

ABSTRACT

Aspects of the invention relate to intracranial electrodes and methods for implanting and using intracranial electrodes. In one particular example, an intracranial electrode includes a shaft having a distal contact surface adapted to electrically contact a surface of a patient&#39;s brain, a head associated with the shaft, and threads adapted to fix the electrode with respect to the patient&#39;s skull. This electrode may have an adjustable length adapted to change a contact force of the distal contact surface against the surface of the brain by adjusting the length of the electrode.

TECHNICAL FIELD

The present invention relates to intracranial electrodes and methods forimplanting and using intracranial electrodes. These electrodes andmethods are particularly well suited for neurostimulation systems andmay also be used in electroencephalography and other recording systems,e.g., evoked potential recordings.

BACKGROUND

A wide variety of mental and physical processes are known to becontrolled or influenced by neural activity in the central andperipheral nervous systems. For example, the neural functions in someareas of the brain (e.g., the sensory or motor cortices) are organizedaccording to physical or cognitive functions. Several other areas of thebrain also appear to have distinct functions in most individuals. In themajority of people, for example, the areas of the occipital lobes relateto vision, the regions of the left inferior frontal lobes relate tolanguage, and the regions of the cerebral cortex appear to be involvedwith conscious awareness, memory, and intellect. Because of thelocation-specific functional organization of the brain, in which neuronsat discrete locations are statistically likely to control particularmental or physical functions in normal individuals, stimulating neuronsat selected locations of the central nervous system can be used toeffectuate changes in cognitive and/or motor functions throughout thebody.

In several existing applications, neural functions are treated oraugmented by electrical or magnetic stimulation powered by a neuralstimulator that has a plurality of therapy electrodes and a pulse systemcoupled to the therapy electrodes. The therapy electrodes can beimplanted into the patient at a target site for stimulating the desiredportions of the brain. For example, one existing technique for maskingpain in a patient is to apply an electrical stimulus to a targetstimulation site of the brain. In other applications, stimulation of anappropriate target site in the brain has shown promise for treatingdamage to and disease and disorders of the brain, including damage fromstrokes and treatment of Alzheimer's disease, depression,obsessive-compulsive behavior, and other disorders.

The brain can be stimulated in several known fashions. One type oftreatment is referred to as transcranial electrical stimulation (TES),which involves placing an electrode on the exterior of the patient'sscalp and delivering an electrical current to the brain through thescalp and the skull. TES, however, is not widely used because thedelivery of the electrical stimulation through the scalp and the skullcauses patients a great amount of pain and the electrical field isdifficult to direct or focus accurately.

Another type of treatment is transcranial magnetic stimulation (TMS),which involves using a high-powered magnetic field adjacent the exteriorof the scalp over an area of the cortex. TMS does not cause the painfulside effects of TES. Unfortunately, TMS is not presently effective fortreating many patients because the existing delivery systems are notpractical for applying stimulation over an adequate period of time. TMSsystems, for example, are relatively complex and require stimulationtreatments to be performed by a healthcare professional in a hospital orphysician's office. The efficacy of TMS in longer-term therapies may belimited because it is difficult to (a) accurately localize the region ofstimulation in a reproducible manner, (b) hold the device in the correctposition over the cranium for the requisite period, and (c) providestimulation for extended periods of time.

Another device for stimulating a region of the brain is disclosed byKing in U.S. Pat. No. 5,713,922, the entirety of which is incorporatedherein by reference. King discloses a device for cortical surfacestimulation having electrodes mounted on a paddle that is implantedunder the skull of the patient. These electrodes are placed in contactwith the surface of the cortex to create “paresthesia,” which is avibrating or buzzing sensation. Implanting the paddle typically requiresremoval of a relatively large (e.g., thumbnail-sized or larger) windowin the skull via a full craniotomy. Craniotomies are performed under ageneral anesthetic and subject the patient to increased chances ofinfection.

A physician may employ electroencephalography (EEG) to monitor neuralfunctions of a patient. Sometimes this is done alone, e.g., indiagnosing epileptic conditions, though it may also be used inconjunction with neurostimulation. Most commonly, electroencephalographyinvolves monitoring electrical activity of the brain, manifested aspotential differences at the scalp surfaces, using electrodes placed onthe scalp. The electrodes are typically coupled to anelectroencephalograph to generate an electroencephalogram. Diagnosis ofsome neurological diseases and disorders, e.g., epilepsy, may best beconducted by monitoring neural function over an extended period of time.For this reason, ambulatory electroencephalography (AEEG) monitoring isbecoming more popular. In AEEG applications, disc electrodes are appliedto the patient's scalp. The scalp with the attached electrodes may bewrapped in gauze and the lead wires attached to the electrodes may betaped to the patient's scalp to minimize the chance of displacement.

EEG conducted with scalp-positioned electrodes requires amplification ofthe signals detected by the electrodes. In some circumstances, it can bedifficult to pinpoint the origin of a particular signal because of thesignal dissipation attributable to the scalp and the skull. For moreprecise determinations, EEG may be conducted using “deep brain”electrodes. Such electrodes extend through the patient's scalp and skullto a target location within the patient's brain. Typically, these deepbrain electrodes comprise lengths of relatively thin wire that areadvanced through a bore through the patient's skull to the desiredlocation. If the electrodes are to be monitored over an extended periodof time, the electrodes typically are allowed to extend out of thepatient's skull and scalp and are coupled to the electroencephalographusing leads clipped or otherwise attached to the electrodes outside thescalp. To avoid shifting of the electrodes over time, the electrodestypically are taped down or held in place with a biocompatiblecementitious material. The patient's head typically must be wrapped ingauze to protect the exposed electrodes and the associated leads, andthe patient is uncomfortable during the procedure. This may be suitablefor limited testing purposes-deep brain encephalography typically islimited to tests conducted in hospital settings over a limited period oftime, usually no more than a few days—but could be problematic forlonger-term monitoring, particularly in nonclinical settings.

Screws have been used to attach plates or the like to patients' skulls.FIG. 1, for example, schematically illustrates a conventional cranialreconstruction to repair a fracture 50 or other trauma. In thisapplication, a plate 60 is attached to the outer cortex 12 of the skull10 by cortical bone screws 62. The plate 60 spans the fracture 50,helping fix the skull in place on opposite sides of the fracture 50. Ascan be seen in FIG. 1, the screws 62 do not extend through the entirethickness of the skull. Instead, the screws 62 are seated in the outercortex 12 and do not extend into the cancellous 18 or the inter cortex14. In some related applications, the screws 62 may be longer and extendinto or even through the cancellous 18. Physicians typically takesignificant care to ensure that the screws 62 do not extend through theentire thickness of the skull, though, because penetrating the skull canincrease the likelihood of trauma to or infection in the patient'sbrain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a conventional cranialreconstruction.

FIG. 2A is a schematic view in partial cross section of an intracranialelectrode in accordance with one embodiment of the invention implantedin a patient.

FIG. 2B is a schematic top elevation view of the implanted intracranialelectrode of FIG. 2A.

FIG. 3A is a schematic illustration in partial cross section of anintracranial electrode in accordance with another embodiment of theinvention implanted in a patient.

FIG. 3B is a schematic top elevation view of the implanted intracranialelectrode of FIG. 3A.

FIG. 4A is a schematic illustration in partial cross section of anintracranial electrode in accordance with yet another embodiment of theinvention implanted in a patient.

FIG. 4B is a side view of a dielectric member of the electrode of FIG.4A.

FIG. 5 is a schematic illustration in partial cross section of anintracranial electrode in accordance with still another embodiment ofthe invention implanted in a patient.

FIG. 6A is a schematic illustration in partial cross section of anintracranial electrode in accordance with a further embodiment of theinvention implanted in a patient.

FIG. 6B is a schematic top elevation view of the implanted intracranialelectrode of FIG. 6A.

FIG. 7 is a schematic illustration in partial cross section of anintracranial electrode in accordance with still another embodiment ofthe invention implanted in a patient.

FIG. 8 is a schematic side view of a broken-away portion of a patient'sskull in which an intracranial electrode in accordance with anotherembodiment of the invention has been implanted.

FIG. 9 is a schematic partial cross-sectional view taken along line 9-9of FIG. 8.

FIG. 10 is an isolation view of a portion of the implanted electrode ofFIG. 9.

FIG. 11 is a perspective view of selected components of the intracranialelectrode of FIGS. 8-10.

FIG. 12 is a schematic illustration in partial cross section of anintracranial electrode in accordance with yet another embodiment of theinvention implanted in a patient.

FIG. 13 is a schematic illustration in partial cross section of anintracranial electrode in accordance with one more embodiment of theinvention implanted in a patient.

FIG. 14 is a schematic illustration in partial cross section of anintracranial electrode in accordance with a further embodiment of theinvention implanted in a patient.

FIG. 15 is a schematic partial cross-sectional view of the intracranialelectrode of FIG. 13 with a retaining collar of the electrode in aradially compressed state.

FIG. 16 is a schematic partial cross-sectional view of the intracranialelectrode of FIG. 14 with the retaining collar in a radially expandedstate.

FIG. 17 is a schematic illustration in partial cross section of a deepbrain intracranial electrode in accordance with an alternativeembodiment of the invention implanted in a patient.

FIG. 18 is a schematic illustration in partial cross section of a deepbrain intracranial electrode in accordance with still another embodimentof the invention implanted in a patient.

FIG. 19 is a schematic overview of a neurostimulation system inaccordance with a further embodiment of the invention.

FIG. 20 is a schematic overview of a neurostimulation system inaccordance with another embodiment of the invention.

FIG. 21 is a schematic illustration of one pulse system suitable for usein the neurostimulation system of FIG. 17 or FIG. 18.

FIG. 22 is a schematic top view of the array of electrodes in FIG. 17.

FIGS. 23-26 are schematic top views of alternative electrode arrays inaccordance with other embodiments of the invention.

DETAILED DESCRIPTION

A. Overview

Various embodiments of the present invention provide intracranialelectrodes and methods for implanting and using intracranial electrodes.It will be appreciated that several of the details set forth below areprovided to describe the following embodiments in a manner sufficient toenable a person skilled in the art to make and use the disclosedembodiments. Several of the details and advantages described below,however, may not be necessary to practice certain embodiments of theinvention. Additionally, the invention can also include additionalembodiments that are not described in detail with respect to FIGS. 1-24.

One embodiment of the invention provides an intracranial electrode thatincludes a shaft, a head, and threads. The shaft includes a distalcontact surface adapted to electrically contact a surface of a patient'sbrain. The head is associated with the shaft and is sized to bepositioned subcutaneously adjacent the patient's skull. This canameliorate the difficulties associated with electrodes that protrudethrough the patient's scalp, including irritation of the skin anddiscomfort. The threads are carried by at least one of the head and theshaft and may be adapted to fix the electrode with respect to thepatient's skull. This intracranial electrode has an adjustable lengthadapted to change a contact force of the distal contact surface againstthe surface of the brain by adjusting the length of the electrode. Inone adaptation of this embodiment, the intracranial electrode includesan adjustment means that is adapted to adjust the length of theelectrode.

Another embodiment of the invention provides an intracranial electrodethat includes an electrically conductive member, a dielectric member,and an anchor. The electrically conductive member has a blunt contactsurface and the dielectric member has an interior in which theelectrically conductive member is received. The anchor may be carried bythe electrically conductive member or the dielectric member. The anchoris adapted to anchor at least one of the electrically conductive memberand the dielectric member with respect to a patient's skull adjacent abrain surface such that the contact surface of the electricallyconductive member is in electrical contact with the brain surface andthe dielectric member electrically insulates the skull from theelectrically conductive member.

Another embodiment of the invention provides a neurostimulator system.This neurostimulator system includes an intracranial electrode, a lead,and a pulse system. The intracranial electrode may take the form of oneof the preceding embodiments. In one particular implementation, theintracranial electrode includes a shaft including a distal contactsurface adapted to electrically contact a surface of a patient's brain;a head associated with the shaft, with the head being sized to bepositioned subcutaneously adjacent the patient's skull; and threadscarried by at least one of the head and the shaft, with the threadsbeing adapted to fix the electrode with respect to the patient's skull.The lead is adapted to be subcutaneously implanted beneath the patient'sscalp. The lead has a first portion, which is adapted to be electricallycoupled to the contact surface, and an electrically insulatedimplantable length. The pulse system is adapted to be implanted in thepatient's body at a location spaced from the electrode. The pulse systemis operatively coupled to the electrode via the lead to deliver anelectrical stimulus to the brain via the electrode. If so desired, anarray of such intracranial electrodes may be employed and the pulsesystem may be adapted to generate an electrical potential between theelectrodes in the array.

A method of implanting an intracranial electrode in accordance withanother embodiment of the invention involves advancing a threadedelectrode through a patient's skull until a contact surface of theelectrode is in atraumatic contact with a surface of the patient'sbrain. The threaded electrode is electrically coupled to a lead. A headof the electrode and the length of the lead are covered with thepatient's scalp, thereby enclosing the electrode. An electrical stimulusis delivered to the patient's brain via the electrode. This electricalstimulus may be generated by a pulse system electrically coupled to theelectrode by the lead. In one adaptation of this embodiment, the methodmay also include adjusting a length of the electrode, e.g., to adjustthe force of the contact surface of the electrode against the surface ofthe brain.

For ease of understanding, the following discussion is subdivided intothree areas of emphasis. The first section discusses certainintracranial electrodes; the second section relates to selectembodiments of neurostimulation systems; and the third section outlinesmethods in accordance with other embodiments of the invention.

B. Intracranial Electrodes

FIGS. 2-15 illustrate intracranial electrodes in accordance with variousembodiments of the invention. Like reference numbers are used throughoutthese figures to designate like or analogous elements.

FIGS. 2A-B illustrate an intracranial electrode 100 in accordance withone embodiment of the invention. This electrode 100 includes a head 102attached to a threaded shaft 110. The head 102 and shaft 110 may beintegrally formed of an electrically conductive material, e.g., titaniumor another biocompatible, electrical conductive metal. The head 102 mayinclude one or more slots 104, an allen head recess (not shown), orother structure (e.g., a square drive or TORX™ drive recess) adapted tofacilitate turning the electrode 100. As the electrode 100 is turned,the threads 112 of the threaded shaft 110 will advance a generallydistally positioned contact surface 115 of the electrode 100 toward thedura mater 20. The length of the shaft 110 may be selected so that thecontact surface 115 of the electrode 100 electrically engages thesurface of the dura mater 20 without causing undue harm to the duramater 20 or the underlying cerebral cortex. The contact surface 115 maycomprise a relatively blunt end to reduce trauma to the dura mater andthe underlying brain tissue 25.

In one embodiment, the intracranial electrode 100 is adapted to beelectrically connected to a pulse system (1050 in FIG. 18, for example),as described below. The electrode 100 may be connected to the pulsesystem in any desired fashion. In the illustrated embodiment, theelectrode 100 is coupled to such a pulse system by means of anelectrical lead 120. The electrical lead 120 shown in FIGS. 2A and 2Bcomprises an elongated, subcutaneously implantable body 124, which mayhave an insulative sheath. An electrically conductive ring or washer 122may be attached to an end of the body 124. In one embodiment, anopposite end of the body 124 is physically attached to a component ofthe pulse system. In other embodiments, the leads may be operativelyconnected to one or more components of the pulse system without beingphysically attached thereto, e.g., using a transmitter and antenna or amagnetic coupling. Embodiments of pulse systems incorporating suchwireless links are disclosed in U.S. Patent Application Publication No.US 2002/0087201, the entirety of which is incorporated herein byreference.

The head 102 of the electrode 100 is adapted to be implantedsubcutaneously beneath the patient's scalp 30 (shown schematically inFIG. 2A). As explained below, the electrode 100 may be used to deliveran electrical signal to the brain tissue 25 adjacent the contact surface115. At higher stimulus levels, electrical contact between the patient'sscalp 30 and the head 102 of the electrode 100 may be uncomfortable forthe patient. If so desired, the scalp 30 may be electrically insulatedfrom the head 102. This may be accomplished by applying on the head 102a quantity of a dielectric, biocompatible, cementitious material (notshown), which may be cured or dried in place. In another embodiment, thehead 102 may be covered with a separate cap 130 (shown in dashed linesin FIG. 2A) formed of a dielectric material, e.g., a dielectric,biocompatible plastic, that may be glued, press-fit, or otherwiseattached to the head 102 and/or the lead 120.

The dimensions of the electrode 100 can be varied to meet various designobjectives. In one embodiment, however, the electrode 100 is longer thanthe thickness of the patient's skull. More specifically, the head 102 isadapted to be seated at an extracranial subcutaneous site while thethreaded shaft 110 is only slightly longer than the skull thickness atthe intended treatment site. Lengths on the order of 4-50 mm, forexample, may be appropriate in certain applications. The diameter of thehead 102 and the threaded shaft 110 may also be varied. For mostapplications, shafts 110 having diameters (typically excluding the widthof the threads 112) of no greater than 4 mm will suffice. Shaftdiameters of about 1-4 mm are likely, with diameters of 1.5-2.5 mm beingwell suited for most applications. FIGS. 2A-B illustrate an electrode100 having a constant diameter shaft 110, but it should be understoodthat the shaft diameter may vary. For example, the shaft 110 may taperdistally to improve the ability of the shaft 110 to be self-tapping. Thehead 102 typically will have a larger diameter than an adjoining portionof the shaft 110. (It should be recognized that FIGS. 2-15 are not drawnto scale. In particular, the aspect ratio of the electrodes issignificantly reduced to better illustrate certain functional aspects ofthe designs.)

FIG. 3A illustrates an intracranial electrode 150 in accordance withanother embodiment of the invention. This intracranial electrode 150 issimilar in many respects to the intracranial electrode 100 of FIGS.2A-B. For example, the electrode 150 includes an electrically conductivethreaded shaft 110 defining a blunt, atraumatic contact surface 115adjacent a distal end.

The connection of the electrode 150 to the lead 160 in FIGS. 3A-Bdiffers somewhat from the connection of the electrode 100 and lead 120in FIGS. 2A-B, however. In FIGS. 2A-B, the electrode 100 is electricallycoupled to the lead 120 by compressively engaging the electricallyconductive ring 122 of the lead 120 between the electrode head 102 andthe skull 10. In FIGS. 3A-B, the electrode 150 includes a head 152including slots 154 or other structure for engaging a screwdriver,wrench, or the like. The head 152 is adapted to engage a cap 162 carriedby the lead 160 that electrically couples the body 164 of the lead 160to the head 152 of the electrode 150. In the illustrated embodiment, thecap 162 comprises a dielectric body (e.g., a dielectric plastic materialwith some resilience) having an electrically conductive inner surface163, which may be provided by coating an interior surface of the cap 162with a metal. In one embodiment, the cap 162 is adapted to resilientlydeform to be press-fitted on the head 152. The body 164 of the lead 160may be coupled to the electrically conductive inner surface 163 of thecap 162, thereby providing an electrical pathway between the electrode150 and a pulse system (not shown) operatively coupled to the lead 160.

In one embodiment, the cap 162 is sized to be subcutaneously implantedbeneath the patient's scalp 30. In the illustrated embodiment, the head152 and the cap 162 both extend outwardly beyond the outer cortex 12 ofthe patient's skull 10. In another embodiment (not shown) some or all ofthe length of the head 152 and/or the cap 162 may be countersunk into arecess formed through the outer cortex 12 and/or an outer portion of thecancellous 18. This can improve patient comfort, which can be useful ifthe intracranial electrode 150 is intended to be implanted permanentlyor for an extended period of time.

FIGS. 4A-B schematically illustrate aspects of an intracranial electrode200 in accordance with another embodiment. The electrode 200 maycomprise an electrically conductive inner portion 205 and anelectrically insulative outer portion 206. In the illustratedembodiment, the electrically conductive portion 205 of the electrode 200includes a head 202 and a threaded shaft 210 defining a contact surface215 for electrically contacting the patient's dura mater 20. Theseelements of the electrode 200 and their electrical connection to thelead 120 are directly analogous to the electrode 100 shown in FIGS.2A-B. The electrically insulative outer portion 206 of the electrode 200shown in FIG. 4A comprises a dielectric member 240 that is disposedbetween the threaded shaft 210 and the patient's skull 10. As shown inFIG. 4B, this dielectric member 240 may take the form of a taperedsleeve. The sleeve 240 may have an upper ring-like portion 242 and aplurality of deformable flanges 244 extending distally therefrom. Theflanges 244 may be adapted to be urged outwardly into compressivecontact with a bore formed in the patient's skull 10 when the threadedshaft 210 is advanced into the interior of the sleeve 240. Although notshown in FIG. 4B, ribs or teeth may be provided on the exterior surfacesof the flanges 244 to further anchor the sleeve 240 in the cancellous18. In one embodiment, the sleeve 240 is formed of a dielectric plasticand the threads of the threaded member 210 may be self-tapping in theinner wall of the sleeve 240.

When implanted in a skull 10 as shown in FIG. 4A, the dielectric sleeve240 will electrically insulate the skull 10 from the electricallyconductive shaft 210 of the electrode 200. (The sleeve 240 need notcompletely electrically isolate the skull and shaft 210; it merelyserves to reduce electrical conduction to the skull 10.) As explainedbelow, some embodiments of the invention employ an array comprising aplurality of intracranial electrodes implanted at various locations in apatient's skull 10. The use of a dielectric member such as thedielectric sleeve 240 can help electrically isolate each of theelectrodes 200 from other electrodes 200 in the array (not shown). If sodesired, the electrode 200 may be provided with a dielectric cap 230sized and shaped to be implanted subcutaneously beneath the patient'sscalp 30 (not shown in FIG. 4A). Much like the cap 130 of FIG. 2A, thiscap 230 may electrically insulate the patient's scalp from theelectrically conductive head 202. This may further improve electricalisolation of the electrodes 200 in an array.

FIG. 5 illustrates an intracranial electrode 250 in accordance with yetanother embodiment of the invention. This electrode 250 includes anelectrically conductive shaft 260 electrically coupled to asubcutaneously implantable head 262 and a distally positioned contactsurface 265. The shaft 260 is received in the interior of an externallythreaded dielectric layer 280. The shaft 260 may be operatively coupledto the dielectric layer 280 for rotation therewith as the electrode isthreadedly advanced through the patient's skull 10. In one embodiment,this may be accomplished by a spline connection between the shaft 260and the dielectric layer 280. In other embodiments, the dielectric layer280 may be molded or otherwise formed about the shaft 260.

In one particular embodiment, the dielectric layer 280 comprises anelectrically insulative ceramic material. In another embodiment, thedielectric layer 280 comprises an electrically insulative plastic orother biocompatible polymer that has sufficient structural integrity toadequately anchor the electrode 250 to the skull 10 for the duration ofits intended use. If so desired, the dielectric layer 280 may be porousor textured to promote osseointegration of long-term implants. Forshorter-term applications, the dielectric layer 280 may be formed of orcovered with a material that will limit osseointegration.

In each of the preceding embodiments, the intracranial electrode 100,150, 200, or 250 has a fixed length. In the embodiment shown in FIGS.2A-B, for example, the distance between the base of the head 102 and thecontact surface 115 remains fixed. When the threaded shaft 110 is sunkinto the skull 10 to a depth sufficient to compress the conductive ring122 of the lead 120 between the head 102 and the skull 10, this willalso fix the distance from the exterior surface of the outer cortex 12of the skull 10 to the contact surface 115. The thickness of the skull10 can vary from patient to patient and from site to site on a givenpatient's skull. Hence, the pressure exerted by the contact surface 115against the dura mater 20 will vary depending on the thickness of theskull. If the electrode 100 is selected to be long enough to makeadequate electrical contact with the dura mater adjacent the thickestsite on a skull, the pressure exerted by the contact surface 115 againstthe dura mater 20 may cause undue damage at sites where the skull isthinner. Consequently, it can be advantageous to provide a selection ofelectrode sizes from which the physician can choose in selecting anelectrode 100 for a particular site of a specific patient's skull.

FIGS. 6-12 illustrate embodiments of electrodes with adjustable lengths.FIGS. 6A-B, for example, illustrate an intracranial electrode 300 thatis adapted to adjust a distance between the outer surface of the skull10 and a contact surface 315 of the electrode 300. This, in turn,enables the contact force between the contact surface 315 and thesurface of the dura mater 20 to be varied without requiring multipleelectrode lengths.

The intracranial electrode 300 of FIGS. 6A and 6B includes a probe orshaft 310 that has a blunt distal surface defining the contact surface315 of the electrode 300. The shaft 310 has a proximal end 312 that mayinclude a torque drive recess 314 or the like to facilitate rotation ofthe shaft 310 relative to a head 320 of the electrode 300. At least aportion of the length of the shaft 310 is externally threaded. In theillustrated embodiment, the shaft has an externally threaded proximallength and an unthreaded surface along a distal length.

The head 320 of the electrode 300 comprises a body 322 and a tubularlength 324 that extends from the body 322. The body 322 may be adaptedto be rotated by hand or by an installation tool. In one embodiment thebody 322 is generally hexagonal to facilitate rotation with anappropriately sized wrench. In the particular embodiment shown in FIGS.6A-B, the body 322 has a pair of recesses 323 in its outer face sizedand shaped to interface with a dedicated installation tool (not shown)having projections adapted to fit in the recesses 323. If so desired,the installation tool may be a torque wrench or other tool adapted tolimit the amount of torque an operator may apply to the head 320 of theelectrode 300 during installation. The tubular length 324 may beexternally threaded so the head 320 may be anchored to the skull 10 byscrewing the tubular length 324 into the skull 10.

The head 320 includes an internally threaded bore 326 that extendsthrough the thickness of the body 322 and the tubular length 324. Thebore 326 has threads sized to mate with the external threads on theshaft 310. If so desired, a biocompatible sealant (e.g., a length ofpolytetrafluoroethylene tape) may be provided between the threads of thebore 326 and the threads of the shaft 310 to limit passage of fluids orinfectious agents through the bore 326.

Rotation of the shaft 310 with respect to the head 320 will, therefore,selectively advance or retract the shaft 310 with respect to the head320. This will, in turn, increase or decrease, respectively, thedistance between the lower face 323 of the head body 322 and the contactsurface 315 of the shaft 310. As suggested in FIG. 6A, this may beaccomplished by inserting a tip 344 of a torque driver 340 into thetorque drive recess 314 in the shaft 310 and rotating the torque driver340. The tip 344 of the torque driver 340 may be specifically designedto fit the torque drive recess 314. In the embodiment shown in FIGS.6A-B, the torque drive recess 314 is generally triangular in shape andis adapted to receive a triangular tip 344 of the torque driver 340. Ifso desired, the torque driver 340 may comprise a torque wrench or thelike that will limit the maximum torque and operator can apply to theshaft 310 of the electrode 300.

If so desired, the torque driver 340 may include graduations 342 toinform the physician how far the shaft 310 has been advanced withrespect to the head 320. As noted below, in certain methods of theinvention, the thickness of the skull at the particular treatment sitemay be gauged before the electrode 300 is implanted. Using thisinformation and the graduations 342 on the torque driver 340, thephysician can fairly reliably select an appropriate length for theelectrode 300 to meet the conditions present at that particular site.

In the embodiment shown in FIGS. 6A-B, the head 320 and the shaft 310are both formed of an electrically conductive material. The conductivering 122 of the lead 120 may be received in a slot formed in the lowerface 323 of the body 322. Alternatively, the ring 122 may be internallythreaded, permitting it to be threaded over the external threads of thetubular length 324 before the head 320 is implanted. If so desired, thering 122 can instead be compressively engaged by the lower face 323 ofthe head 320 in a manner analogous to the engagement of the head 102with the ring 122 in FIG. 2A, for example.

In another embodiment, the head 320 is formed of a dielectric material,such as a dielectric ceramic or plastic. This may necessitate adifferent connection between the lead 120 and the shaft 310, such as byelectrically contacting the lead 120 to the proximal end 312 of theshaft 310. Employing a dielectric head 320 can help electricallyinsulate the skull 10 from the electrodes 300, improving signal qualityand reducing interference between the various electrodes 300 in anarray, as noted above.

FIG. 7 schematically illustrates an intracranial electrode 350 inaccordance with a further embodiment of the invention. The electrode 350includes a shaft or probe 360 having a proximal end 362 and a distallylocated contact surface 365. The shaft 360 may include a first threadedportion 360 a and a second threaded portion 360 c. In the embodimentshown in FIG. 7, the first and second threaded portions 360 a and 360 care separated by an unthreaded intermediate portion 360 b. In analternative embodiment, the two threaded portions 360 a and 360 cdirectly abut one another.

The intracranial electrode 350 of this embodiment also includes a head370 having an internally threaded bore 376 extending through itsthickness. The threads of the bore 376 are adapted to mate with thethreads of the first threaded portion 360 a. By rotating the shaft 360with respect to the head 370 (e.g., with a screwdriver 340), thedistance between the head 370 and the contact surface 365 can beadjusted in much the same manner described above in connection withFIGS. 6A-B.

The head 320 of the electrode 300 in FIGS. 6A-B has an externallythreaded tubular length 324 that extends into the skull 10 and helpsanchor the electrode 300 to the skull 10. The shaft 310 may then movewith respect to the skull by rotating the shaft 310 with respect to thehead 320. In the embodiment shown in FIG. 7, the head 370 is notdirectly anchored to the skull 10. Instead, the threads of the secondthreaded portion 360 c are adapted to threadedly engage the skull 10 toanchor the electrode 350 with respect to the skull 10 and the head 370is attached to the first threaded portion 360 a of the shaft 360. In oneembodiment, the shaft 360 may be threaded into a pilot hole in the skull10. Once the shaft 360 is positioned at the desired depth, the head 370may be screwed onto the first threaded portion 360 a of the shaft 360 tohelp fix the shaft 360 with respect to the skull and provide a lesstraumatic surface to engage the patient's scalp (not shown) when thescalp is closed over the electrode 350. In another embodiment, thelength of the electrode 350 may first be adjusted by rotating the shaft360 with respect to the head 370. Once the electrode 350 has the desiredlength, the shaft 360 may be advanced into the skull 10. The shaft 360may be graduated to facilitate adjustment to the appropriate length. Ifso desired, the first threaded portion 360 a may be threaded in adirection opposite the second threaded portion 360 c and/or the pitch ofthe threads in the first threaded portion 360 a may be different fromthe pitch of the threads in the second threaded portion 360 c.

In the embodiment of FIG. 7, the shaft 360 of the electrode 350 extendsthrough the dura mater 20 and the contact surface 365 of the electrode350 is in direct contact with the cerebral cortex of the patient'sbrain. This is simply intended to illustrate one alternativeapplication. In other embodiments, the length of the electrode 350 maybe selected so that the contact surface 365 electrically contacts thedura mater 20 without extending therethrough, much as illustrated inFIG. 6A, for example.

FIGS. 8-11 illustrate an intracranial electrode 400 in accordance withanother embodiment of the invention. The intracranial electrode 400includes a shaft or probe 410 that is slidably received by a head 420.The shaft 410 comprises an electrically conductive material and definesan electrical contact surface 415, e.g., on its distal end.

In the preceding embodiments, some or a majority of the head of theelectrode extends outwardly beyond the outer surface of the skull 10. Inthe particular implementation shown in FIGS. 8-10, the head 420 isreceived entirely within the thickness of the skull 10. It should beunderstood, though, that this is not necessary for operation of thedevice, and this is shown simply to highlight that the position of thehead 420 with respect to the skull 10 can be varied. In anotherembodiment, at least a portion of the head 420 extends outwardly beyondthe outer surface of the skull 10.

The head 420 includes a base 430 and an actuator 422. The base 430includes an externally threaded body 432 and a tubular length 434 thatextends from the body 432. A portion of the tubular length 434 carriesexternal threads 436. The tubular length 434 may also include one ormore locking tabs 440, each of which includes an actuating surface 442.

The actuator 422 has an internally threaded bore 424 that is adapted tomatingly engage the threads 436 on the base 430. Rotating the actuator422 with respect to the base 430 in a first direction will advance theactuator 422 toward the actuating surface 442 of each of the tabs 440.The actuator 422 may urge against the actuating surfaces 442, pushingthe tabs 440 inwardly into engagement with the shaft 410. This will helplock the shaft 410 in place with respect to the base 430. Rotating theactuator 422 in the opposite direction will allow the tabs 440 toresiliently return toward a rest position wherein they do not brakemovement of the shaft 410. The force with which the shaft 410 engagesthe dura mater 20 (not shown) then can be adjusted to a desired level bymoving the shaft 410 with respect to the base 430. When the shaft 410 isin the desired position, the actuator 422 may be moved into engagementwith the tabs 440 to hold the shaft 410 in the desired position.

FIG. 12 illustrates an adjustable-length intracranial electrode 450 inaccordance with another embodiment. The intracranial electrode 450includes an axially slidable probe or shaft 452 and a head 460. The head460 includes a body 462 and an externally threaded tubular length 464.The tubular length 464 includes an axially extending recess 466 sized toslidably receive a portion of the shaft 452. An O-ring 465 or the likemay provide a sliding seal between the head 460 and the shaft 452.

The contact surface 455 of the shaft 452 is pushed against the surfaceof the dura mater 20 with a predictable force by means of a spring 454received in the recess 466. In FIG. 12, the spring 454 is typified as acompressed coil spring formed of a helically wound wire or the like. Inthis embodiment, an electrical contact 469 of the lead 468 may beelectrically coupled to the wire of the spring 454. Electrical potentialmay then be conducted to the shaft 452 by the wire of the spring 454.

In another embodiment (not shown), the spring 454 comprises a compressedelastomer, which may take the form of a column that fills some or all ofthe diameter of the recess 466. The elastomer may comprise abiocompatible polymeric material, for example. In such an embodiment,the elastomer may be electrically conductive, e.g., by filling apolymeric material with a suitable quantity of a conductive metal powderor the like. In another embodiment, one or more wires may be embedded inthe elastomeric material to conduct an electrical signal across theelastomer to the shaft 452.

In the illustrated embodiment and the alternative embodiment wherein thespring 454 comprises an elastomer, the head 460 may be formed of adielectric material, helping electrically insulate the skull 10 from theshaft 452. In an alternative embodiment, the head 460 may be formed ofan electrically conductive material. Even though the other structuralelements of the electrode 450 may remain largely the same, this wouldavoid the necessity of having the lead 468 extend through the head 460;an electrically conducive ring 122 or the like instead may be employedin a manner analogous to that shown in FIG. 6A, for example.

FIG. 13 depicts and adjustable-length intracranial electrode 475 inaccordance with a different embodiment. Some aspects of the intracranialelectrode 475 are similar to the intracranial electrode 450 shown inFIG. 12. In particular, the intracranial electrode 475 includes anaxially slidable probe or shaft 480 that is slidably received in anaxially extending recess 488 in a tubular length 492 of a head 490. Aproximal face of the body 490 may include a pair of tool-receivingrecesses 494, which may be analogous to the tool-receiving recesses 323noted above in connection with FIG. 6A, to aid in the installation ofthe body 490. If so desired, one or more seals may be provided betweenthe shaft 480 and the body 490. In the embodiment shown in FIG. 13, thebody 490 carries a first O-ring 493 and the shaft 480 and carries asecond O-ring 484 sealed against the interior of the recess 488. TheseO-rings may also serve as abutments to limit axial travel of the shaft480 in the recess 488.

The contact surface 481 of the shaft 480 is pushed against the surfaceof the dura mater 20 with a predictable force by means of a spring 486.The spring 486 may be substantially the same as the spring 454 shown inFIG. 12, and the various materials suggested above for the spring 454may also be employed in the spring 486 of FIG. 13.

In FIG. 12, the spring 454 provides the electrical connection betweenthe lead 468 and the shaft 452. In the embodiment of FIG. 13, however,the lead 496 may be connected directly to the shaft 480 through a lumen495 in the body 490. This lumen 495 is sized to slidably receive areduced-diameter neck 482 of the shaft 480. As the body 490 is screwedinto the skull 10 and moves toward the brain 25, contact between theshaft 480 and the dura mater 20 will urge the shaft 480 upwardly, movingthe neck 482 upwardly within the lumen 495.

The electrode 475 of FIG. 13 may facilitate delivering a highlyreproducible contact force of the contact surface 481 of the shaft 480against the dura mater 20. The position of the reduced-diameter neck 482of the shaft 480 within the lumen 495 will vary in a fixed relationshipwith the force exerted on the spring 486 by the shaft 480. Since theforce of the shaft 480 against the spring 486 is essentially the same asthe force of the shaft 480 against the dura mater 20, knowing theposition of the neck 482 within the lumen 495 can give the operator anindication of the force exerted against the dura mater 20. In oneparticular embodiment, the interior of the lumen 495 may be graduated tomark off the depth of the neck 482 in the lumen 495. In anotherembodiment, the body 490 may be driven into the skull 10 until theheight of the neck 482 in the lumen 495 reaches a predetermined point,e.g., when the top of the neck 482 is flush with the top of the body490.

FIG. 14 illustrates an intracranial electrode 500 in accordance withstill another embodiment of the invention. This electrode 500 includesan electrically conductive probe or shaft 510 having a head 512 and acontact surface 515. A radially compressible retaining collar 540extends along a portion of the length of the shaft 510. As shown in FIG.15, the retaining collar 540 may be adapted to assume a radially reducedconfiguration in response to a compressive force, indicatedschematically by the arrows F. This compressive force F may be generatedby collapsing the retaining collar 540 and restraining it in the lumenof an introducing sheath (not shown) sized to be received in a borethrough the skull 10. When this force F is removed (e.g., by retractingthe introducing sheath), the retaining collar 540 may expand radiallyoutwardly away from the shaft 510, as illustrated in FIG. 16.

To implant the electrode 500 in the skull 10, the shaft 510 may beadvanced into a bore in the skull until the contact surface 515 exertsthe desired contact force against the dura mater 20. Once the shaft 510is in the desired position, the compressive force F on the collar 540may be released, allowing the collar 540 to expand outwardly intocompressive engagement with the lumen of the bore in the skull 10. Thiswill help hold the electrode 500 in place with respect to the skullwithout requiring permanent anchoring of the shaft 510 to the skull 10.

The shaft 510 may be electrically coupled to a pulse system (not shown)by a lead 520. The lead 520 may include a cap 522 having an electricallyconductive inner surface 524 coupled to a body 526 of the lead. The lead520 may be analogous to the lead 160 shown in FIGS. 3A-B. Any othersuitable electrical connection between the shaft 510 and the pulsesystem may be employed.

In one embodiment, the collar 540 comprises a dielectric material. Thiswill help electrically insulate the skull 10 from the shaft 510. Inanother embodiment, the collar 540 is electrically conductive and thelead 520 may be electrically coupled to the shaft 510 via the collar540.

In the embodiment shown in FIG. 14, the shaft 510 may have a length onlya little longer than the thickness of the patient's skull 10 and thecontact surface 515 may be relatively blunt. Such a design is useful forrelatively atraumatic contact with the dura mater 20. In anotherembodiment suggested in dashed lines in FIGS. 15 and 16, the electrode500 may instead have a substantially longer shaft 510 a and a relativelysharp contact surface 515 a. Such an embodiment may be useful fordirectly stimulating a particular location within the cerebral cortex orsome other location within the deeper tissues of the brain.

FIG. 17 schematically illustrates how certain principles of theinvention can be embodied in a deep brain intracranial electrode 550.The electrode 550 generally includes a threaded shaft 560 having a head562. The head 562 may be coupled to a pulse system or a sensing unit (asdescribed below) via a lead 160 in the same manner lead 160 is attachedto the head 152 of electrode 150 in FIGS. 3A-B. (Like reference numbersare used in these figures to indicate like elements.) The electrode 550also includes an elongate conductive member 570 that extends inwardlyfrom the skull 10 to a selected target site 28. The conductive member570, which may comprise a length of a conductive wire, may beelectrically shielded by a dielectric sheath along much of its lengthand have an exposed, electrically conductive tip 574.

In use, the conductive member 570 may be slid freely through a pilothole 11 formed through the skull to position the tip 574 at the targetsite 28 in a known manner. The pilot hole 11 may be larger than theconductive member 570 or be tapped to receive the threads of the shaft560. With the conductive member 570 in place, the shaft 560 may bethreaded into the pilot hole 11, crimping the conductive member 570against an interior of the pilot hole 11. This will fix the conductivemember 570 in place. If so desired, a proximal length 572 of theconductive member 570 may extend outwardly of the skull and be held inplace by the head 562. The threads of the threaded shaft 560 may alsocut through the dielectric sheath of the conductive member 570 as theshaft 560 is screwed into place, making electrical contact with theconductive wire therein.

FIG. 18 schematically illustrates a deep brain intracranial electrode600 in accordance with an alternative embodiment of the invention. Thiselectrode 600 includes a head 610 having a threaded shaft 620 with anaxially-extending opening 622 extending through the length of the head610. The head 610 may also include a gimbal fitting 630 adapted toslidably receive a length of a conductive member, which may comprise thesame type of elongate conductive member 570 discussed above inconnection with FIG. 17.

The gimbal fitting 630 is adapted to allow an operator greater controlover the placement of the electrically conductive tip 574 of theconductive member 570. In use, the tip 574 of the conductive member 570will be threaded through an opening in the gimbal fitting 630. Bypivoting the gimbal fitting 630 with respect to the threaded shaft 620of the head 610, the angular orientation of the conductive member 570with respect to the pilot hole 11 in the skull 10 can be accuratelycontrolled. Once the operator determines that the conductive member 570is at the appropriate angle, e.g., using a surgical navigation systemsuch as that noted below, the operator may advance the conductive member570 to position the conductive tip 574 at the target site 28. Once thetip 574 is in position, the cap 162 of a lead 160 may be press-fitted onthe body 610 of the electrode 600. This will crimp the proximal length572 of the connective member 570 between the body 610 and the conductiveinner surface 163 of the cap 162, providing an effective electricalconnection between the conductive member 570 and the body 164 of thelead 160.

C. Systems Employing Intracranial Electrodes

FIG. 19 is a schematic illustration of a neurostimulation system 1000 inaccordance with one embodiment of the invention. This neurostimulationsystem 1000 includes an array 1010 of intracranial electrodes and aninternally implantable pulse system 1050. The array 1010 of electrodesmay employ one or more electrodes in accordance with any one or more ofthe embodiments described above in connection with FIGS. 2-18 or anyother suitable design. In the particular implementation depicted in FIG.19, the array 1010 (shown schematically in FIG. 20) includes a firstimplantable intracranial electrode 100 a and a second implantableintracranial electrode 100 b, each of which may be substantially thesame as the electrode 100 shown in FIGS. 2A-B. These electrodes 100 band 100 b extend through the skull 10 into contact with the dura mater20 at two spaced-apart locations.

The pulse system 1050 may be implanted in the body of the patient P at alocation remote from the array 1010 of electrodes 100. In the embodimentshown in FIG. 19, the pulse system 1050 is adapted to be implantedsubclavicularly. In the alternative embodiment shown in FIG. 20, thepulse system 1050 is adapted to be implanted in a recess formed in thepatient's skull 10. In either embodiment, each of the electrodes 100 inthe array 1010 is electrically coupled to the pulse system 1050 by meansof a separate lead (120 in FIGS. 2A-B) having an elongate,subcutaneously implantable body 124. Hence, electrode 100 a is coupledto the pulse system 1050 by the elongate body 124 a of a first lead andthe other electrode 100 b is coupled to the pulse system 1050 by theelongate body 124 b of another lead. In one embodiment, the elongatebodies 124 a-b are combined into a single subcutaneously implantablecable or ribbon.

FIG. 21 schematically illustrates one pulse system 1050 suitable for usein the neurostimulation system 1000 shown in FIG. 19. The pulse system1050 generally includes a power supply 1055, an integrated controller1060, a pulse generator 1065, and a pulse transmitter 1070. The powersupply 1055 can be a primary battery, such as a rechargeable battery orother suitable device for storing electrical energy. In alternativeembodiments, the power supply 1055 can be an RF transducer or a magnetictransducer that receives broadcast energy emitted from an external powersource and converts the broadcast energy into power for the electricalcomponents of the pulse system 1050.

In one embodiment, the controller 1060 includes a processor, a memory,and a programmable computer medium. The controller 1060, for example,can be a computer, and the programmable computer medium can be softwareloaded into the memory of the computer and/or hardware that performs therequisite control functions. In an alternative embodiment suggested bydashed lines in FIG. 21, the controller 1060 may include an integratedRF or magnetic controller 1064 that communicates with an externalcontroller 1062 via an RF or magnetic link. In such a circumstance, manyof the functions of the controller 1060 may be resident in the externalcontroller 1062 and the integrated portion 1064 of the controller 1060may comprise a wireless communication system.

The controller 1060 is operatively coupled to and provides controlsignals to the pulse generator 1065, which may include a plurality ofchannels that send appropriate electrical pulses to the pulsetransmitter 1070. The pulse generator 1065 may have N channels, with atleast one channel associated with each of N electrodes 100 in the array1010. The pulse generator 1065 sends appropriate electrical pulses tothe pulse transmitter 1070, which is coupled to a plurality ofelectrodes 1080. In one embodiment, each of these electrodes is adaptedto be physically connected to the body 124 of a separate lead, allowingeach electrode 1080 to electrically communicate with a single electrode100 in the array 1010 on a dedicated channel of the pulse generator1065. Suitable components for the power supply 1055, the integratedcontroller 1060, the pulse generator 1065, and the pulse transmitter1070 are known to persons skilled in the art of implantable medicaldevices.

As shown in FIG. 20, the array 1010 of electrodes 100 in FIG. 19comprises a simple pair of electrodes 100 a and 100 b implanted in thepatient's skull at spaced-apart locations. FIGS. 23-26 illustratealternative arrays that may be useful in other embodiments. In FIG. 23,the array 1010 a includes four electrodes 100 arranged in a rectangulararray. The array 1010 b of FIG. 24 includes sixteen electrodes 100, alsoarranged in a rectangular array. The array 1010 c shown in FIG. 25includes nine electrodes 100 arranged in a radial array. FIG. 26illustrates an array 1010 d that includes four electrodes 100 x arrangedin a rectangular pattern and a fifth electrode 100 y at a locationspaced from the other four electrodes 100 x. In using such an array, thefour proximate electrodes 100 x may be provided with the same polarityand the fifth electrode 100 y may have a different polarity. In someembodiments, the housing (1052 in FIG. 19) of the pulse system 1050 mayserve the function of the fifth electrode 100 y. The precise shape,size, and location of the array 1010 and the number of electrodes 100 inthe array 110 can be optimized to meet the requirements of anyparticular application.

The electrodes 100 of these arrays 1010 may be provided with electricalsignals in a variety of different manners. In some circumstances, oneelectrode 100 or a subset of the electrodes 100 may have one electricalpotential and a different electrode 100 or subset of the electrodes 100(or, in some embodiments, the housing 1052 of the pulse system 1050) mayhave a different electrical potential. U.S. patent application Ser. No.09/978,134, entitled “Systems and Methods for Automatically OptimizingStimulus Parameters and Electrode Configurations for Neuro-Stimulators”and filed 15 Oct. 2001 (the entirety of which is incorporated herein byreference), suggests ways for optimizing the control of the electricalpulses delivered to the electrodes 100 in an array 1010. The methods andapparatus disclosed therein may be used to automatically determine theconfiguration of therapy electrodes and/or the parameters for thestimulus to treat or otherwise effectuate a change in neural function ofa patient.

The preceding discussion focuses on use of intracranial electrodes(e.g., electrodes 100, 150, 200, 250, 300, 350, 400, 450, 475, 500, 550,or 600) in neurostimulation systems. In an alternative application, theintracranial electrodes may be used to monitor electrical potentials inelectroencephalography. A suitable electroencephalograph may incorporatea system similar to the neurostimulation system 1000 shown in FIG. 19,but a sensing unit (not shown) may be used in place of the pulse system1050. Suitable components for such a sensing unit are known to thoseskilled in the art of electroencephalography.

D. Methods

As noted above, other embodiments of the invention provide methods ofimplanting an intracranial electrode and/or methods of installing aneurostimulation system including an implantable intracranial electrode.In the following discussion, reference is made to the particularintracranial electrode 100 illustrated in FIGS. 2A-B and to theneurostimulation system 1000 shown in FIG. 19. It should be understood,though, that reference to this particular embodiment is solely forpurposes of illustration and that the methods outlined below are notlimited to any particular apparatus shown in the drawings or discussedin detail above.

As noted above, implanting conventional cortical electrodes typicallyrequires a full craniotomy under general anesthesia to remove arelatively large (e.g., thumbnail-sized or larger) window in the skull.Craniotomies are performed under a general anesthetic and subject thepatient to increased chances of infection.

In accordance with one embodiment of the present invention, however, thediameter of the electrode shaft 110 is sufficiently small to permitimplantation under local anesthetic without requiring a craniotomy. Inthis embodiment, a relatively small (e.g., 4 mm or smaller) pilot holemay be formed through at least part of the thickness of the patient'sskull adjacent a selected stimulation or monitoring site of the brain.When implanting the electrode 100 of FIGS. 2A-B, it may be advantageousto extend the pilot hole through the entire thickness of the skull. Careshould be taken to avoid undue trauma to the brain in forming the pilothole. In one embodiment, an initial estimate of skull thickness can bemade from MRI, CT, or other imaging information. A hand-held drill maybe used to form a bore shallow enough to avoid extending through theentire skull. A stylus may be inserted into the pilot hole to confirmthat it strikes relatively rigid bone. The drill may then be used todeepen the pilot hole in small increments, checking with the stylusafter each increment to detect when the hole passes through thethickness of the inner cortex 14 of the skull 10. If so desired, thestylus may be graduated to allow a physician to measure the distance tothe springy dura mater and this information can be used to select anelectrode 100 of appropriate length or, if an adjustable-lengthelectrode (e.g., electrode 300 of FIGS. 6A-B) is used, to adjust theelectrode to an appropriate length.

The location of the pilot hole (and, ultimately the electrode 100received therein) can be selected in a variety of fashions. U.S. PatentApplication Publication No. US 2002/0087201 and U.S. application Ser.No. 09/978,134 (both of which are incorporated hereinabove), forexample, suggest approaches for selecting an appropriate stimulationsite. When the desired site has been identified, the physician can borethe pilot hole to guide the contact surface 115 of the electrode 100 tothat site. In one embodiment, the physician may use anatomicallandmarks, e.g., cranial landmarks such as the bregma or the sagittalsuture, to guide placement and orientation of the pilot hole. In anotherembodiment, a surgical navigation system may be employed to inform thephysician during the procedure. Briefly, such systems may employreal-time imaging and/or proximity detection to guide a physician inplacing the pilot hole and in placing the electrode 100 in the pilothole. In some systems, fiducials are positioned on the patient's scalpor skull prior to imaging and those fiducials are used as referencepoints in subsequent implantation. In other systems, real-time MRI orthe like may be employed instead of or in conjunction with suchfiducials. A number of suitable navigation systems are commerciallyavailable, such as the STEALTHSTATION TREON TGS sold by MedtronicSurgical Navigation Technologies of Louisville, Colorado, US.

Once the pilot hole is formed, the threaded electrode 100 may beadvanced along the pilot hole until the contact surface 115 electricallycontacts a desired portion of the patient's brain. If the electrode 100is intended to be positioned epidurally, this may comprise relativelyatraumatically contacting the dura mater 20; if the electrode is tocontact a site on the cerebral cortex, the electrode will be advanced toextend through the dura mater. The electrodes 100 may also be implantedto a selected depth within the cerebral cortex or at a deeper locationin the brain.

In one embodiment, the length of the electrode 100 is selected (oradjusted for electrode 300, for example) to achieve the desired level ofcontact and the electrode will be advanced until a known relationshipwith the skull is achieved, e.g., when the head 102 compresses thecontact ring 122 of the lead 120 against the exterior of the skull 10.In another embodiment, the thickness of the skull 10 need not be knownto any significant accuracy before the electrode 100 is implanted.Instead, the electrode 100 may be connected, e.g., via the lead 120, toan impedance monitor and the impedance may be monitored as the electrode100 is being implanted. It is anticipated that the measured impedancewill change when the electrode 100 contacts the dura mater 20. Once thiscontact is detected, the physician may advance the electrode a small,fixed distance to ensure reliable electrical contact over time.

As noted above, the electrode 100 may be coupled to a lead 120. Thetiming of this coupling may vary with the nature of the coupling. For alead 120 employing a contact ring 122 or the like positioned below thehead 102, the lead may be coupled to the electrode before the electrodeis introduced into the skull. In other embodiments, the lead (e.g., lead160 of FIGS. 3A-B) may be coupled to the electrode after the electrodeis properly positioned with respect to the selected site of the brain.The lead, or at least a length thereof, may be implanted subcutaneously,e.g., by guiding it through a tunnel formed between the implant site andthe intended site of a subclavicularly implanted pulse system 1050. Thepatient's scalp may then be closed over the head 102 of the electrode100 so the electrode is completely enclosed. This can materially improvepatient comfort compared to more convention systems wherein epilepsymonitoring electrodes or the like extend through the scalp to anextracorporeal connection.

Once the electrode is in place, an electrical stimulus may be deliveredfrom a pulse system 1050 to the patient's brain via the lead 120 and theelectrode 100. In certain embodiments of the invention discussedpreviously, a plurality of electrodes 100 may be implanted in an array(e.g., array 1010, 1010 a, 1010 b, or 1010 c) in the patient's skull andeach of the electrodes 100 may be coupled to the pulse system 1050 by anelectrically separate lead 120. The precise nature of the stimulusdelivered via the electrode(s) 100 can be varied as desired to diagnoseor treat any particular condition. The type and frequency of stimulusmay be selected as outlined in U.S. Patent Application Publication No.US 2002/0087201, for example, and also may be optimized as taught inU.S. application Ser. No. 09/978,134.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense, that is to say, in a sense of “including,but not limited to.” Words using the singular or plural number alsoinclude the plural or singular number, respectively. When the claims usethe word “or” in reference to a list of two or more items, that wordcovers all of the following interpretations of the word: any of theitems in the list, all of the items in the list, and any combination ofthe items in the list.

The above-detailed descriptions of embodiments of the invention are notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example,whereas steps are presented in a given order, alternative embodimentsmay perform steps in a different order. The various embodimentsdescribed herein can be combined to provide further embodiments.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification, unless the above-detailed description explicitlydefines such terms. While certain aspects of the invention are presentedbelow in certain claim forms, the inventors contemplate the variousaspects of the invention in any number of claim forms. Accordingly, theinventors reserve the right to add additional claims after filing theapplication to pursue such additional claim forms for other aspects ofthe invention.

1. An intracranial electrode comprising: a shaft including a distal contact surface adapted to electrically contact a surface of a patient's brain; a head associated with the shaft, the head being sized to be positioned subcutaneously adjacent the patient's skull; and threads carried by at least one of the head and the shaft, the threads being adapted to fix the electrode with respect to the patient's skull; the electrode having an adjustable length adapted to change a contact force of the distal contact surface against the surface of the brain by adjusting the length of the electrode.
 2. The intracranial electrode of claim 1 wherein the shaft is moveable with respect to the head to adjust the length of the electrode.
 3. The intracranial electrode of claim 1 further comprising a spring adapted to move the shaft with respect to the head to adjust the length of the electrode.
 4. The intracranial electrode of claim 1 further comprising a resilient elastomer adapted to move the shaft with respect to the head to adjust the length of the electrode.
 5. The intracranial electrode of claim 1 wherein the shaft and the head include mating threads such that rotation of the shaft relative to the head adjusts the length of the electrode.
 6. The intracranial electrode of claim 1 further comprising an abutment adapted to selectively engage the shaft, the abutment restricting movement of the shaft with respect to the head when engaged with the shaft and permitting movement of the shaft with respect to the head to adjust the length of the electrode when disengaged from the shaft.
 7. The intracranial electrode of claim 1 wherein the shaft has a threaded length and a nonthreaded surface and the distal contact surface comprises at least a portion of the nonthreaded surface of the shaft.
 8. The intracranial electrode of claim 1 wherein at least a portion of the head is adapted to be received in a bore in the patient's skull.
 9. The intracranial electrode of claim 1 wherein the head includes a threaded member adapted to be received in a bore in the patient's skull and the shaft is adapted to move axially along a length of the threaded member.
 10. The intracranial electrode of claim 1 wherein the head includes a threaded member adapted to be received in a bore in the patient's skull, the threaded member having internal threads that mate with external threads on the shaft such that rotation of the shaft relative to the head adjusts the length of the electrode.
 11. The intracranial electrode of claim 1 wherein the shaft is electrically conductive and the head is electrically insulative.
 12. The intracranial electrode of claim 1 wherein the shaft is electrically conductive and the head includes a length adapted to be received in the patient's skull and electrically insulate the patient's skull from the shaft.
 13. The intracranial electrode of claim 1 wherein the head is electrically conductive, further comprising a dielectric cap adapted to electrically insulate the patient's scalp from the head.
 14. The intracranial electrode of claim 1 further comprising a lead electrically coupled to the contact surface and adapted to carry an electrical signal to or from the contact surface.
 15. The intracranial electrode of claim 1 further comprising a lead electrically coupled to the contact surface and adapted to carry an electrical signal subcutaneously from a pulse system to the contact surface.
 16. The intracranial electrode of claim 1 further comprising a lead electrically coupled to the contact surface and adapted to be subcutaneously implanted adjacent a skull.
 17. The intracranial electrode of claim 1 further comprising a lead electrically coupled to the contact surface, the lead being adapted for subcutaneous implantation and to deliver an electrical signal to or from the contact surface.
 18. An intracranial electrode comprising: a shaft including a distal contact surface adapted to electrically contact a surface of a patient's brain; a head associated with the shaft, the head being sized to be positioned subcutaneously adjacent the patient's skull; threads carried by at least one of the head and the shaft, the threads being adapted to fix the electrode with respect to the patient's skull; and adjustment means adapted to adjust a length of the electrode to change a contact force of the distal contact surface against the surface of the brain.
 19. An intracranial electrode, comprising: an electrically conductive member having a blunt contact surface; a dielectric member having an interior in which the electrically conductive member is received; and an anchor carried by the electrically conductive member or the dielectric member and adapted to anchor at least one of the electrically conductive member and the dielectric member with respect to a patient's skull adjacent a brain surface such that the contact surface is in electrical contact with the brain surface and the dielectric member electrically insulates the skull from the electrically conductive member.
 20. The intracranial electrode of claim 19 further comprising a dielectric cap adapted to electrically insulate the patient's scalp from the electrically conductive member.
 21. The intracranial electrode of claim 19 further comprising a cap including an electrically conductive portion adapted to be electrically coupled to the electrically conductive member and a dielectric portion adapted to electrically insulate the patient's scalp from the electrically conductive member.
 22. The intracranial electrode of claim 19 further comprising an electrical lead electrically coupled to the electrically conductive member.
 23. The intracranial electrode of claim 19 wherein the anchor comprises an external surface of the dielectric member, an external surface of the electrically conductive member includes threads, and the dielectric member is adapted to urge the external surface outwardly into engagement with a lumen of a bore in the skull when the electrically conductive member is threaded into the interior of the dielectric member.
 24. The intracranial electrode of claim 19 wherein the anchor is carried by the dielectric member and the electrically conductive member is adapted to move the contact surface with respect to the dielectric member.
 25. The intracranial electrode of claim 19 wherein the anchor comprises threads carried by a threaded portion of the dielectric member and the electrically conductive member is slidable along a length of the threaded portion.
 26. The intracranial electrode of claim 25 further comprising a spring adapted to push the contact surface of the electrically conductive member against the brain surface.
 27. The intracranial electrode of claim 19 wherein the anchor comprises threads carried by the dielectric member and the electrically conductive member is fixed with respect to the dielectric member.
 28. The intracranial electrode of claim 19 wherein the anchor comprises threads carried by the dielectric member and the electrically conductive member is splined for rotation with the dielectric member.
 29. The intracranial electrode of claim 19 wherein the anchor member comprises an engagement surface of the dielectric member and the dielectric member comprises a compressible collar adapted to expand outwardly to engage the engagement surface with a lumen of a bore in the patient's skull. 30-51. (canceled) 